Monoculture

A MONOCULTURE REFERS to the repeated cultivation of a single crop on an area of land and describes the practice of relying on a population of plants or animals derived from a single genotype or a very narrow genetic base. This implies an agricultural practice without any crop rotation or mixed intercropping. In forestry it refers to planting of single species tree crops instead of encouraging a diverse canopy of trees, so that biodiversity that would provide a suitable habitat for a number of different species cannot be provided. Monoculture is characterized by the absence of diversity and can occur on various scales: a plot of ground, a natural community, a landscape, or a large geographic region. Its correlative, polyculture, indicates that two are more crops or herds are part of the system, which is also scale dependent. For instance, a peanut field might be regarded as a monoculture, but fruit trees planted as wind breaks make the system a polyculture, as does any system that grazes animals on field stubble to refertilize the soil.

Monocultures in agriculture have been introduced to maximize the productivity of a single crop. Monocultures perpetuate and even exacerbate the general impacts of agriculture on ecosystems, such as narrowing the genetic base of the system by replacing the natural vegetation, destroying habitats for the natural fauna, increasing vulnerability to erosion by the decline of humus content, aggregating stabilities of soils as a consequence of soil tillage and conversion of virgin land, interrupting and simplifying the natural food web, and underutilizing the variety of niches of an ecosystem with subsequent loss of homeostasis and self regulation mechanisms of the system.

The practice of replanting the same species over years leads to additional yield declines that are due to autotoxity and monoculture injury. Explanations for this include: growing the same crop species in the same soil year after year leads selectively to a deficiency in one or more plant nutrients that would not be limiting to other crop species; the crop plant builds up a toxicity to itself, leading to self-inhibition; or, growing the same crop species in the same soil in subsequent years enriches the soilborne pathogens of the roots of those crops. This helps lead to complete crop failures. The most wellknown among these is the Irish potato famine in the beginning of the 20th century, which was due to the expansion of the fungus Phytophtora infestans in potato monocultures.

Nevertheless, about 70 percent of the world's food crops are grown in monocultures. The world's agricultural landscapes are planted mostly with some 12 species of grain crops—especially wheat and corn—23 vegetable crop species, and about 35 fruit and nut crop species; that is, no more than 70 plant species cover approximately 1,440 million hectares of presently cultivated land in the world. Reliance on such a small number of crops has reduced the global genetic diversity to an extent that there are fewer and fewer varieties to draw upon for adaptive genes. For instance, in the United States, 60–70 percent of the total bean area is planted with two to three bean varieties, 72 percent of the potato area with four varieties, and 53 percent of the cotton area with three varieties.

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